Pressure-relief valve devices of various designs are widely used to protect mining equipment against excessive pressure and especially hydraulic props which are exposed to high forces by the roof of a mine working. Known devices are of small compact design and are coupled to the equipment to connect to a working chamber thereof so as to be exposed to the pressure fluid therein. Normally the valves of such a device remains closed but excessive pressure causes the valve to open to permit fluid to pass through the device and escape to relieve the excess pressure. Known types of devices are described in U.S. Pat. Nos. 4,284,101 and 4,313,463 and in German Patent Specification No. 1,303,157.
It is desirable that when the valve of the device opens and fluid flows through the device the quantity of through flow per unit of time (Q) is such that the pressure of the fluid should not rise (.DELTA.P) more than 20% above the setting pressure of the valve (P) and that the opening pressure rise should be kept as low as possible. In addition, the closing pressure should be as close as possible to the opening pressure to provide a good response. Those requirements are in contradistinction to one another and it has been found that the characteristics of the compression spring which holds the piston or valve member in a closed position are decisive in determining the overall response of the valve device. A hard spring imposes a short stroke or displacement on the valve member and produces a short closure time and a high closure force. A sudden pressure rise however only creates a slight increase in the quantity of through flow (Q) and an undesired pressure rise (.DELTA.P) occurs. A soft spring on the other hand allows a longer stroke of the valve member and a progressive increase in the through flow quantity to minimize the pressure rise. However, this is offset by a low closure force which can cause fluttering or oscillations with increased wear and locking and damage can occur to the spring.
In the known devices the spring abuts on a dish-like piston or pressure piece which transfers force to the valve member itself. When the valve opens fluid passes at high pressure and speed from radial bores in a stem of the valve member and into an annular chamber beneath the pressure piece which leads via outflow to the exterior. The pressure fluid impinges on the plain annular surface at the underside of the pressure piece and creates wear.
A general object of the present invention is to provide an improved form of valve device. Another object is to provide a valve device wherein the opening and closing pressures are close to one another, yet the pressure rise (.DELTA.P) during through flow is minimised. A further object is to provide a valve device which has a long service life.